Field of the Invention
The present invention relates to a fixing device, which is mounted on a copier or a printer using an electrophotographic recording technique, and fixes an unfixed image formed on a recording medium, onto the recording medium.
Description of the Related Art
In recent years, a fixing device (hereinafter referred to as a “film fixing device”) using a cylindrical film (belt) has become widespread. The film fixing device can have a smaller heat capacity, and thus have an advantage of being able to suppress power consumption. Many of the film fixing devices are configured to drive a roller in contact with an outer surface of the film, thereby rotatably driving the film. This is because such a configuration can simplify a structure. Such a configuration requires a back-up member disposed in contact with an inner surface of the film in a fixed manner, for pressing the film against the roller. In a film fixing device using a ceramic heater, this heater corresponds to the back-up member. Further, to reduce a rotational load of the film, grease is applied to the inner surface of the film. Meanwhile, components such as a roller are mounted in a U-shaped groove provided in a frame of the fixing device (see Japanese Patent Application Laid-Open No. 2011-75860).
In fixing devices, toner offset often occurs as a problem. As one of solutions to this problem, there is a fixing device that has a function of forming an electric field between a film surface and a roller surface using a direct-current power supply, so that the toner offset is suppressed by the action of the electric field. In this case, the film is provided with a conductive layer.
Meanwhile, if the film or roller is reduced in diameter to downsize the fixing device, the distance between a shaft (made of metal) of the roller and an end surface of the film becomes small. It is also conceivable that leakage may occur between the shaft (made of metal) of the roller or a metal bearing, and the end surface of the film, because a high voltage is applied between the film and the roller to suppress the toner offset. In addition, the grease applied to the inner surface of the film may protrude from the end surface of the film. The protruding grease may further reduce the distance relative to the shaft of the roller or to the bearing, and the leakage may occur more easily if the grease is conductive. If the leakage occurs, a potential difference between the film and the roller (a pressure roller) disappears, which reduces an effect of suppressing the offset.